1. Field of the Invention
The present invention is directed to chemical sensors. More particularly, the present invention is directed to chemical sensors and sensor arrays that employ a microcantilever to recognize chemical or biological agents exposed to the sensor.
2. Background Information
Various types of sensors have been used to detect the presence of chemical or biological agents. One class of sensor measures the swelling a common organic polymer undergoes as vapor molecules from the agent exposed to the sensor partition into the polymer. For example, agents having a Hildebrand solubility parameter similar to that of the polymer cause the polymer to swell. The degree of swelling can be measured to determine the concentration of a specific chemical or biological agent exposed to the sensor. However, the approach of measuring polymer swelling is not always useful when the agent has a solubility parameter that is far from that of the polymer because the polymer swelling is very small and thus hard to accurately measure.
Agent absorption into the polymer can be measured in a number of other ways as well. For example, one approach is to use a scanning force microscope (SFM) cantilever to measure increases in mass associated with absorption of the agent into the polymer. Other types of mass-sensitive resonant devices include the quartz-crystal microbalance, surface-acoustic-wave, and flexural-plate-wave resonator. In this type of sensor, an SFM-type cantilever is coated with a chemically or biologically active material. The cantilever is then driven into oscillation at a resonance frequency. As the agent molecules bind or absorb into the active layer on the cantilever, the mass of the vibrating cantilever increases. This increase can be measured as a shift in the cantilever vibration frequency or amplitude. One problem with these sensors is that the external circuitry required to drive the cantilever and measure the vibration can be cumbersome, expensive and power demanding.
Another approach for measuring polymer swelling is to observe the bending of a cantilever that is coated on one side with the polymer. As the polymer expands, the cantilever bends to accommodate the expansion. The difficulty with this approach is that a uniform polymer coating is difficult to apply and the polymer-cantilever interface can sometimes fail to statically maintain the stress differential.
Still another approach for measuring polymer swelling is to use partially conductive polymers to measure changes in conductivity of the polymer created as the polymer absorbs molecules from the agent. Partially conductive polymers can be created by incorporating carbon black particles or other conductive materials into the polymer. Upon incorporation of the carbon black particles, the normally non-conductive polymer becomes partially conductive. The composite is then deposited onto electrodes.
Exposure of the polymer/carbon composite to a chemical or biological agent causes the composite to swell. This swelling creates a drop in conductivity of the composite. Electronics connected to the electrodes can be used to measure the drop in conductivity.
This approach also suffers from problems. For example, the sensor can suffer from drift and variable sensitivity because the carbon particles may rearrange as the composite swells and contracts thus returning the polymer to a different nominal conductivity. Furthermore, the carbon particles themselves may absorb the chemical or biological agents thus extending the recovery time of the sensor. Additionally, all of the above approaches suffer from selectivity problems because the sensors measure only one factor affected by the analyte and two different analytes may affect the factor in similar manners.
Thus, there is a need for an improved sensor that can accurately identify a wide range of chemical or biological agents. There is also a need for an improved sensor that is durable and has a short recovery time. There is a further need for a sensor having improved selectivity.